The present invention relates to a magnetic toner used for developing electrostatic latent images in image forming methods, such as electrophotography and electrostatic recording, or an image forming method of toner jetting scheme, and a process cartridge containing the magnetic toner.
Demands for apparatus utilizing electrophotography have been extended to printers as output means for computers and facsimile apparatus in addition to conventional use as copying machines for reproducing originals. Further, in recent years, increased demands are given to more compact and higher-speed output machines. For complying with such demands, toners have been required to achieve improvements in various items, inclusive of developing performance, low-temperature fixability, prevention of image deterioration in low temperature/low humidity environments, and long-term continuous image forming performances in high temperature/high humidity environments.
More specifically, a toner applicable to a higher-speed printing machine is required to securely retain a uniformly high triboelectric charge on a developing sleeve and be transferred for development onto a photosensitive drum. As a measure for providing an increased toner chargeability, it has been proposed to make the toner shape close to a sphere, and processes for production of such spherical toners by spraying particulation, dissolution in solutions and polymerization have been disclosed in Japanese Laid-Open Patent Application (JP-A) 3-84558, JP-A 3-229268, JP-A 4-1766 and JP-A 4-102862.
On the other hand, in the conventional pulverization toner production process, toner ingredients, such as a binder resin, a colorant and a release agent, are dry-blended and melt-kneaded by conventional kneading apparatus, such as a roll mill, an extruder, etc. After being solidified by cooling, the kneaded product is pulverized and classified by a pneumatic classifier, etc. to adjust a particle size necessary for a toner, and then further blended with external additives, such as a flowability-improving agent and a lubricant, as desired, to formulate a toner used for image formation.
As the pulverization means, various pulverizers have been used, and a jet air stream-type pulverizer, particularly an impingement-type pneumatic pulverizer, is used for pulverization of a coarsely crushed toner product.
In such an impingement-type pneumatic pulverizer, a powdery feed material is ejected together with a high-pressure gas to impinge onto an impingement surface and be pulverized by the impact of the impingement. As a result, the pulverized toner is liable to be indefinitely and angularly shaped, and have a relatively low triboelectric chargeability due to abundant presence of magnetic iron oxide on the toner particle surface, thus being liable to result in a lower image density due to a lower triboelectric charge in a high temperature/high humidity environment.
Spherical toner particles having a smooth and less-angular surface have smaller contact areas with a developing sleeve and the photosensitive drum and exhibit a smaller attachment force onto these members, thus providing a toner showing good developing and transfer efficiencies.
JP-A 2-87157 and JP-A 10-097095 have proposed a method of subjecting toner particles produced through the pulverization process to mechanical impact by a hybridizer to modify the particle shape and surface property, thereby providing an improved transferability. According to this method, more spherical toner particles can be obtained compared with those obtained by the pneumatic pulverization method, thus acquiring a higher triboelectric chargeability. However, as the impact application step is inserted as an additional step after pulverization, the toner productivity and production cost are adversely affected, and further a fine powder fraction is increased due to the surface treatment, so that the toner chargeability is liable to be only locally introduced to result in image defects such as fog in some cases.
JP-A 6-51561 has disclosed a method of sphering toner particles by surface melting in a hot air stream. According to the toner treatment by this method, however, the toner surface composition is liable to be changed to result in an unstable charge increase rate at the time of triboelectrification. As a result, in case where the opportunity of friction is increased as in a high-speed machine, the charge difference is liable to increase between a freshly supplied portion of toner and a remaining portion of toner on the sleeve, thereby causing negative ghost or positive ghost (i.e., a potion of photosensitive drum having provided a solid black image leaves a lower-density portion or a higher-density portion in a subsequent solid halftone image as illustrated in FIGS. 7 and 8, respectively). Further, as a result of high-temperature heat application, a wax component contained in the toner is liable to exude to the toner particle surface, thus adversely affecting anti-blocking property and storability in a high temperature/high humidity environment. Further, Japanese Patent (JP-B) 3094676 has disclosed a toner having a specific dielectric loss obtained through surface modification by treatment in a hot air stream or application of a continuous impact force exerted by a rotating or vibrating stirring impacting member. According to this method, however, magnetic iron oxide exposed to the toner particle surface is positively covered with the resinous toner components, thus failing to function as charge leakage sites for preventing excessive charge to provide an appropriate charge level.
Thus, the toner particle surface state significantly affects the toner chargeability and further the developing performance of the toner. JP-A 6-342224 has disclosed a method of affixing resin fine particles onto base toner particles under application of a mechanical impact force, thereby controlling the resin and wax contents at the toner particle surfaces. According to this method of affixing the resin fine particles under application of a mechanical impact, the resin layer is liable to peel off the toner particle surface, so that it is difficult to uniformly treat the entire toner particles.
JP-A 11-194533 has proposed a method of measuring an absorbance of toner particles dispersed in an ethanol/water mixture solution having a specific volumetric ratio of 26/73 as a measure for evaluating the state of presence of magnetic material on the toner particle surface and controlling the absorbance within a specific range to control the toner chargeability and suppress the toner melt-sticking onto the photosensitive member. According to this method, however, the toner state is checked only at one point, and the entire behavior and distribution of toner particles cannot be evaluated, thus leaving a room for improvement.
EP-A 1058157 has disclosed a magnetic toner comprising toner particles produced by suspension polymerization and having a low surface-exposed iron content. The toner, however, exhibits a low methanol wettability and has left a room for improvement regarding the charging stability in continuous image formation.
A generic object of the present invention is to provide a magnetic toner having solved the above-mentioned problems.
A more specific object of the present invention is to provide a magnetic toner exhibiting a quick chargeability and capable of suppressing fog and ghost.
Another object of the present invention is to provide a magnetic toner causing little image scattering and exhibiting a high dot reproducibility.
A further object of the present invention is to provide a magnetic toner capable of suppressing image defects such as white streaks caused by developing failure.
According to the present invention, there is provided a magnetic toner, comprising: magnetic toner particles each comprising at least a binder resin and a magnetic iron oxide; wherein the magnetic toner shows a wettability characteristic in methanol/water mixture liquids such that it shows a transmittance of 80% for light at a wavelength of 780 nm at a methanol concentration in a range of 65-75% and a transmittance of 20% at a methanol concentration in a range of 66-76%.
In a preferred embodiment, the magnetic toner has a weight-average particle size X in a range of 4.5-11.0 xcexcm and contains at least 90% by number of particles having a circularity Ci according to formula (1) below of at least 0.900 with respect to articles of 2 xcexcm or larger therein,
Ci=L0/Lxe2x80x83xe2x80x83(1), 
wherein L denotes a peripheral length of a projection image of an individual particle, and L0 denotes a peripheral length of a circle having an identical area as the projection image; and the magnetic toner contains a number-basis percentage Y (%) of particles having Cixe2x89xa70.950 within particles of 3 xcexcm or larger satisfying:
Yxe2x89xa7Xxe2x88x920.645xc3x97exp 5.51xe2x80x83xe2x80x83(2). 
The present invention further provides a process cartridge, detachably mountable to a main assembly of an image forming apparatus and comprising: at least an image-bearing member for bearing an electrostatic latent image thereon, and a developing means containing the above-mentioned magnetic toner for developing the electrostatic latent image on the image-bearing member with the magnetic toner to form a toner image.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.